The terahertz (THz) region of the electromagnetic spectrum extends from 100 GHz to 10 THz (where 1 THz corresponds to a wavelength of 300 microns and photon energy of 4.1 meV). This region, alternatively called the far-infrared, lies below visible and infrared wavelengths and above microwave wavelengths. From RF waves through X-rays, this narrow portion of the electromagnetic spectrum is the least developed and therefore the least understood scientifically and technologically. Recently, however, there have been important advances using electronic and optical techniques to generate and detect THz radiation. See, for example, D. Mittleman, “Sensing with Terahertz Radiation” (Springer-Verlag, 2003) and K. Sakai, “Terahertz Optoelectronics” (Springer-Verlag, 2005).
What technological benefits are to be derived from advancing our ability to generate and detect THz radiation? It is the ability of terahertz radiation to “see” through materials that are opaque at other frequencies that has caused the most excitement for safety, security, and non-invasive diagnostic applications. For example, FIG. 1 shows terahertz transmission as a function of frequency through typical packaging materials. The transmission of cardboard and Styrofoam at THz frequencies is substantial whereas at infrared and optical frequencies these materials are opaque. These materials would also be transparent to X-rays but THz radiation has the advantage of being non-ionizing. Microwaves would also be easily transmitted through these materials, but THz has additional advantages in several important respects. For imaging applications, the shorter wavelength of THz radiation in comparison to microwaves leads to a higher spatial resolution (sub-millimeter for THz in comparison to cm for microwave).
Furthermore, THz radiation offers the possibility of spectroscopic materials identification. For example, FIG. 2 shows the THz spectrum of a single crystal of the energetic material RDX and the rotational spectrum of N2O gas. For RDX, distinct intermolecular vibrational modes are observed. In combination with the aforementioned ability to “see” through materials which are opaque at other frequencies, THz radiation has the potential to image and spectroscopically identify embedded materials or identify defects in materials. See J. Barber et al., “Temperature-dependent far-infrared spectra of single crystals of high explosives using terahertz time-domain spectroscopy,” Journal of Physical Chemistry A, vol. 109, pp. 3501-3505, Apr. 21, 2005.
However, it is important to emphasize that the examples depicted in FIGS. 1 and 2 were performed in a laboratory environment using the technique of terahertz time-domain spectroscopy (THz-TDS). While THz-TDS is one of the most powerful experimental techniques to investigate terahertz science and technology and the associated potential applications, it is not ideal for many real-world applications primarily because of the low-average power and interferometric-like gated detection which necessitates precision alignment.
More generally, the realization of potential THz applications requires advances in source, component (e.g. lenses, filters, modulators, isolators, etc.), and detector technologies. Indeed, there is an active world-wide effort in each of these areas. One area of research is terahertz quantum cascade lasers; see, for example, B. S. Williams et al., “High-power terahertz quantum-cascade lasers,” Electronics Letters, vol. 42, pp. 89-91, Jan. 19, 2006; G. Fasching et al., “Terahertz microcavity quantum-cascade lasers,” Applied Physics Letters, vol. 87, Nov. 21, 2005; A. Tredicucci et al., “Terahertz quantum cascade lasers: first demonstration and novel concepts,” Semiconductor Science and Technology, vol. 20, pp. S222-S227, July 2005; H. E. Beere et al., “MBE growth of terahertz quantum cascade lasers,” Journal of Crystal Growth, vol. 278, pp. 756-764, May 1, 2005; and A. Tredicucci et al., “Terahertz quantum cascade lasers,” Physica E: Low-Dimensional Systems & Nanostructures, vol. 21, pp. 846-851, March 2004.
Another area of active research is low cost compact detectors such as two-dimensional electron gas plasmon detectors; see, for example, M. Asada, “Proposal and analysis of a semiconductor klystron device using two-dimensional electron gas for terahertz amplification and oscillation,” Japanese Journal of Applied Physics Part 1-Regular Papers Short Notes & Review Papers, vol. 43, pp. 5967-5972, September 2004; M. Asada, “Proposal and analysis of a traveling-wave amplifier in terahertz range using two-dimensional electron gas,” Japanese Journal of Applied Physics Part 1-Regular Papers Short Notes & Review Papers, vol. 43, pp. 1332-1333, April 2004; N. A. Kabir et al., “Terahertz transmission characteristics of high-mobility GaAs and InAs two-dimensional-electron-gas systems,” Applied Physics Letters, vol. 89, Sep. 25, 2006; M. Richter et al., “Model of thermal terahertz light emission from a two-dimensional electron gas,” Physical Review B, vol. 75, March 2007; W. Xu, “Dynamical properties of a terahertz driven two-dimensional electron gas,” Australian Journal of Physics, vol. 53, pp. 87-105, 2000; and K. S. Yngvesson, “Ultrafast two-dimensional electron gas detector and mixer for terahertz radiation,” Applied Physics Letters, vol. 76, pp. 777-779, Feb. 7, 2000.
Our recent work has focused on the development of THz components where our approach has focused on using metamaterials. See H. T. Chen et al., “Identification of a resonant imaging process in apertureless near-field microscopy,” Phys. Rev. Lett., vol. 93, pp. 267401-267404, 2004 and W. J. Padilla et al., “Dynamical electric and magnetic metamaterial response at terahertz frequencies,” Physical Review Letters, vol. 96, pp. 107401/1-4, 2006. Metamaterials (MM) are sub-wavelength composites where the electromagnetic response originates from oscillating electrons in highly conducting metals such as gold or copper, allowing for a design specific resonant response of the electrical permittivity or magnetic permeability. This design flexibility allows for realization of phenomena not available with natural materials.
FIG. 3(a) shows a portion of a planar metamaterial array consisting of 200 nm thick Au split ring resonators (SRRs) fabricated on a GaAs substrate. The individual SRR elements are 36×36 μm, 4 μm line width, 2 μm gap, with a lattice spacing of 50 μm. The underlying principle for creating dynamic or active metamaterials is that the SRR elements behave as a resonant LC circuit (see FIG. 3(b))—the solenoidal structure provides the inductance L and the capacitance C is obtained from the split gap. The array shown in FIG. 3a is designed to be strongly resonant at far-infrared (terahertz) frequencies.
The top portion of FIG. 4 shows the experimentally measured transmission versus frequency for a planar array of Cu split ring resonators fabricated on GaAs. The red curve is the transmission in the unperturbed sample. The resonance labeled ω0 is due to circulating currents while the ω1 resonance is due to an in-phase dipolar response of the carriers. Photoexcitation of carriers in GaAs “short-circuits” the resonant response thereby acting as a switch. The lower portion of FIG. 4 shows electromagnetic simulations of the surface current density confirming this interpretation. Thus, this simple SRR array is an effective notch filter for THz radiation. This array of SRRs can also function as narrowband switch to modulate incident terahertz radiation through dynamic modification of the capacitance. For example, photoexcitation of electrons in the GaAs substrate will shunt the SRR capacitance (see FIG. 3(b)) thereby turning off the resonant response. As FIG. 4 reveals, an incident power of 0.5 mW results in a enhanced transmission and for a power of 1.0 mW (black curve—corresponding a substrate carrier density of 2×1016 cm−3) the resonant response almost entirely vanishes. This result constitutes an optically controlled terahertz switch/modulator and has recently been extended to ultrafast operation (˜10 ps on/off times) using nanostructured substrates having ultrafast carrier recombination times; see H. T. Chen et al., “Ultrafast optical switching of terahertz metamaterials fabricated on ErAs/GaAs Nanoisland Superlattices,” Opt. Lett., vol. 32, pp. 1620-1622, 2007. We have also demonstrated voltage control of the metamaterial response; see H. T. Chen et al., “Active terahertz metamaterial devices,” Nature, vol. 44, pp. 597-600, 2006. The advances in creating functional THz components derive from the ability to design resonant sub-wavelength metamaterial elements.